Patterson et al in the article "A CASE FOR REDUNDANT ARRAYS OF INEXPENSIVE DISKS (RAID)", ACM 1988, Mar. 1988, describe several arrangements for using a plurality of data-storing disk drives. Various modes of operation are described; in one mode the data storage is divided among the several drives to effect a storage redundancy. Data to be stored is partially stored in a predetermined number of the disk drives in the array, at least one of the disk drives storing error detecting redundancies. For example, four of the disk drives may store data while a fifth disk drive may store parity based upon data stored in the four disk drives. Such a redundant array of disk drives may provide high data availability by introducing error detecting redundancy data in one of the disk drives. For example, four data blocks (one data block in each of the four drives) are used to compute an error detecting redundancy, such as a parity value; the computed error detecting redundancy is stored as a fifth block on the fifth drive. All blocks have the same number of data bytes and may be (not a requirement) stored in the five disk drives at the same relative track locations. The five drives form a parity group of drives. If any one of the drives in the parity group fails, in whole or in part, the data from the failing drive can be reconstructed using known error correcting techniques. It is desired to efficiently rebuild and replace the data from the failing disk drive while continuing accessing the drives in the array for data processing operations.
The disk drives in a parity group of drives may act in unison as a single logical disk drive. Such a logical drive has logical cylinders and tracks consisting of like-located cylinders and tracks in the parity group drives. In such array usage, the data being stored is partially stored in each of the data-storing drives in an interleaved manner in a so-called striped mode. Alternately, the disk drives and their data in the parity group may be independently addressable and used in a so-called independent mode.
Whenever one of the disk drives in a single-parity array fails, even though data can be successfully recovered, the fault tolerance to error conditions is lost. To return to a desired fault tolerant state, the failing disk drive should be replaced or repaired and the affected data content rebuilt to the desired redundancy. It is desired to provide control means and methods for effecting such rebuilding of data and its redundancy to remove the error from a partially or wholly failed disk drive in a parity array of disk drives.